Place the white of the separated egg in a small bowl and the yolk in a large bowl. Beat egg white and brush the edges of each pie crust with egg white. Add the second whole egg to the egg yolk; whisk together.
Stir sweet potatoes, pumpkin pie filling, sugar, butter, milk, pumpkin pie spice, and vanilla extract into the egg and yolk. Spoon mixture evenly into the pie crusts. Place pies on a baking sheet.
Bake in the preheated oven until middle of each pie is set, about 15 minutes. Cool pies for 15 minutes.
Place 1 tablespoon whipped cream atop each pie; gently press 1 walnut half into the whipped cream of each pie.